Water is generally the fluid having the best thermal performance in heat-transfer devices operating in closed loop and having to operate in a temperature range between 0° C. and 200° C. The thermal performance of a fluid is determined, for example, by the latent heat (expressed in J/kg) of said fluid in two-phase systems and by the specific heat (expressed in J·kg−1.° C.) in single-phase systems. The higher the latent heat or the higher the specific heat, the better the thermal performance of the fluid.
However, the uses of water are limited because of the problem of freezing which, on the one hand, prevents the thermal system from operating and, on the other hand, destroys the containers in which the water circulates due to the expansion of the water during the freezing phase.
During its solidification, water undergoes an increase in volume due to the arrangement of its crystal lattice. This volume increase is around 7% (the density of ice is 0.93 g/cm3 and the density of water is 0.998 g/cm3 at 0° C.). In a heat-transfer device of the prior art comprising a container in the form of a water-filled pipe, which is quite long so as to behave two-dimensionally, a simple calculation shows that the radius of the pipe increases by around 10% when the water filling the pipe solidifies at 0° C. This strain is generally greater than the yield strain of the various materials constituting the container, or even greater than their strain at break. The consequences of the water solidifying are then of two kinds:                (1) If the strain at break of the material constituting the container is exceeded, the pipe is destroyed and consequently there will be loss of liquid upon thawing;        (2) If the yield strain of the material constituting the container is exceeded, the pipe undergoes plastic deformation, with the diameter increasing and the wall thickness decreasing. During the thawing phase, there is no loss of liquid, but in the case of successive freeze/thaw cycles the pipe progressively deteriorates, leading to its destruction. The same problems affect devices containing any fluid whose volume increases upon solidifying.        
Various solutions have already been envisaged for alleviating these drawbacks. Patent Application FR 2 686 346 teaches the use of a fluid consisting of a mixture of water and an anti-freeze. The anti-freeze reduces the freezing point of the water and prevents ice formation at 0° C. For example, the most common anti-freezes are polyethylene glycol and polypropylene glycol, but other examples are alcohols (methanol, ethanol, etc.). Another solution, disclosed in Patent Application WO 2007/097482, consists in warming the heat-exchange device during operating phases at negative temperatures so as to prevent ice formation.
These two solutions have the drawback of greatly reducing the thermal performance of the heat-exchange device, also called the thermal system. To give an example, the thermal performance of a heat exchanger containing a fluid consisting of a water/ethylene glycol mixture is 30 to 40% lower than that of a heat exchanger comprising a fluid consisting of water and air. This drawback is very problematic in two-phase systems such as heat pipes or two-phase loops for which the use of non-azeotropic binary or ternary mixtures causes distillation phenomena completely incompatible with the thermal performance levels demanded.
The object of the present invention is to provide a heat exchange device that reduces the risk of the container deteriorating when it contains a heat-transfer fluid that increases in volume upon solidifying at low temperature, while still guaranteeing good thermal performance.